restructuring tempdb sql 2014

1. I have two sql instances (named and default) on same DB server machine, server is having 12 logical processors. Do I need to create 12 secondary tempdb files (ndf) on default and named sql instance? or is it sufficient if i create 12 tempdb ndf files on only default sql instance?

2. server is having 12 logical processors and I have created 12 tempdb files with equal size. As per below article is there any changes required on number of secondary tempdb files?

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As a general rule, if the number of logical processors is less than or equal to 8, use the same number of data files as logical processors. If the number of logical processors is greater than 8, use 8 data files and then if contention continues, increase the number of data files by multiples of 4 (up to the number of logical processors) until the contention is reduced to acceptable levels or make changes to the workload/code.
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Since you have 12 Cores and 2 Instances of SQL Server, it would be recommended to go with
1. 8 tempdb data files for Default instance(4 more to have 12 if there are any contentions)
2. 8 tempdb data files for Named instance(4 more to have 12 if there are any contentions)

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Do I need to create 12 secondary tempdb files (ndf) on default and named sql instance?

Yes but as the article says, when having more than 8 processors you should limit the number of files to 8. After that there's almost no gain in having more data files.

As per below article is there any changes required on number of secondary tempdb files?

No. The article is referring to a context of a single SQL Server instance no matter how many SQL Server instances are already installed in the server. Meaning that no matter the configuration in other SQL Server instances it doesn't impact the rest of existing SQL Server instances in the server.